The invention discloses an under-range fast protection system for a half-wavelength line. The system comprises a distance I-segment protection unit for sending an action signal to a control unit whenan I-segment protection action condition is satisfied, a zero negative sequence time difference protection unit for sending an action signal to the control unit when a zero negative sequence time difference protection action condition is satisfied, and the control unit used for rapidly removing a fault near a half-wavelength line outlet when the action signals sent by the distance I-segment protection unit and the zero negative sequence time difference protection unit are received at the same time. The invention also discloses an under-range fast protection method for a half-wavelength line. Distance I-segment protection and zero negative sequence time difference protection are arranged for the half-wavelength line at the same time, and the fault near a half-wavelength line exit is quicklyremoved when the I-segment protection action condition and the zero negative sequence time difference protection action condition are satisfied at the same time. According to the technical scheme, the collection of single end single-end electrical quantity is only needed, the protection delay is small, and the fault near the half-wavelength line exit is quickly removed.